1. Field of the Invention
The present invention relates to a shift control system for a continuously variable transmission having a so-called cruise control system which controls a gear ratio of the continuously variable transmission and thereby maintains a vehicle speed or an inter-vehicle distance in a constant level by means of a vehicle distance control device or a vehicle speed control device.
2. Description of the Prior Art
Japanese Patent Application Laid-open No. 9-202155 discloses a control system of an automatic transmission for a vehicle equipped with a constant speed running control device maintaining the vehicle speed in a constant level. As disclosed, the vehicle distance detecting means detects an inter-vehicle distance(hereinafter referred to as vehicle distance) and then determines a vehicle distance a little longer than a distance where an ordinary driver may start deceleration. When this longer distance is equal to the detected vehicle distance, throttle opening and gear ratio are controlled harmoniously with each other, and thereby a drive point is changed and a sufficient deceleration can be achieved:
However, since the follow-up performance in shifting of the transmission cannot be ensured when the aforementioned conventional constant speed running control device controls a vehicle distance, the vehicle distance may not be kept proper. Particularly, if another vehicle suddenly interposes between the driver""s own vehicle and the preceding vehicle when the driver runs the vehicle while maintaining a proper vehicle distance from the preceding vehicle, the vehicle distance shortens rapidly, and therefore an especially quick follow-up performance in shifting is demanded.
The present invention has been made in view of the aforementioned problem, and its object is to provide a shift control system of a continuously variable transmission which has a vehicle distance control device for controlling a vehicle distance from a preceding vehicle and capable of sufficiently attaining a follow-up performance for maintaining a vehicle distance properly.
To realize the aforementioned object, according to an aspect of the invention, there is provided a shift control system for a continuously variable transmission which has a gear ratio control means for controlling gear ratio of said continuously variable transmission by determining a target gear ratio from a drive point determined on the basis of at least a throttle opening and a vehicle speed, and a vehicle distance control means for maintaining a proper vehicle distance by controlling a throttle opening of an engine, wherein said gear ratio control means includes a shift speed control device for making a shift speed higher than a shift speed established at the time when a vehicle distance is not controlled, when a vehicle distance is controlled by said vehicle control means. Shift speed is the rate at which the gear ratio of the transmission is changed.
The shift speed control device controls a shift speed, i.e., the rate at which the gear ratio is changed, at a higher speed when a vehicle distance is controlled. Thus, the gear ratio can be changed with good response to a shift command issued from the shift control means, and the vehicle distance can be kept proper.
In the preferred embodiment, the shift speed control device is provided with a first shin speed map for setting a shift speed according to a deviation between a target gear ratio and an actual gear ratio when a vehicle distance is not controlled and a second shift speed map for setting a shift speed which is constant and higher than a shift speed at the time when a vehicle distance is not controlled, regardless of a deviation between a target gear ratio and an actual gear ratio, when a vehicle distance is controlled.
Since shift speed is set according to a deviation between a target gear ratio and an actual gear ratio when a vehicle distance is not controlled, a smooth shift control can be performed. In addition, by always increasing the shift speed regardless of the deviation between a target gear ratio and an actual gear ratio when an excellent response is demanded as at the time of controlling a vehicle distance, a sufficient shift response can be ensured and a vehicle distance can be kept proper.
Preferably, the shift speed control device is equipped with a line pressure-correcting device for adding a correction line pressure to a line pressure for an ordinary control when performing a gear ratio control for increasing a shift speed.
In a belt-type continuously variable transmission, for example, when the shift speed control device increases shift speed, a pulley ratio must be quickly changed by supplying a high line pressure to a pulley. Therefore, by adding the correction line pressure to the line pressure for a ordinary control, a sufficient line pressure can be ensured, and the pulley ratio can be changed with good response even if the shift speed is increased.
Further preferably, the shift speed control device is equipped with a shift speed limiter for setting an upper limit of a shift speed at the time of obtaining a deceleration torque by changing a gear ratio to a lower level (side) when the vehicle distance is controlled.
When demand for deceleration torque is generated at the time of controlling the vehicle distance, the deceleration is attempted by closing the throttle opening and thereby shifting gear ratio to a lower level. However, the shift speed control device selects a high shift speed regardless of a deviation between a target gear ratio and an actual gear ratio. In this case, if a gear ratio is shifted to a lower level rapidly and an engine speed becomes less than a revolution speed of a driving wheel, the slow engine speed, i.e., engine braking, can lock the driving wheel. Therefore, by providing the shift speed limiter to prevent excessive engine braking, a smooth deceleration can be performed.
Still further preferably, the gear ratio control means is provided with a target shift map for a vehicle distance control for determining a target gear ratio so that a gear ratio is set to a higher level when the vehicle distance control means controls a vehicle distance, compared to the time when the vehicle distance control means does not control a vehicle distance.
Therefore, since an excessive acceleration/deceleration is not performed at the time of changing speed compared to an ordinary running, a stable running state can be maintained, and thus the driver can enjoy a vehicle distance control without feeling physical discomfort.